Libraries, Abstract Classes, Packages

ADTs

  • Abstract data type → interface/idea

Interface Inheritance for Comparison

  • Interfaces provide way to containerize common functions (OffByN CharacterComparator allows comparison of 2 characters in special way)
  • Interfaces = abstract data type, way to express capability of Objects

Abstract Data Types

  • Abstract Data Type (ADT) defined only by operations, not implementation

Stack ADT

  • push(int x): Puts x on top of stack
  • int pop(): Removes & returns top item from stack

List & Set

Collections

  • Maps also known as associative arrays, associative lists (Lisp), symbol tables, dictionaries (Python)

Java vs Python

  • Every language has features that are "first class citizens"
    • Integral to language, shape approach to solving problems in that language
    • For first class data structures, often special syntax for creating & using such objects
  • In Java, programmer can decide which implementation of abstract data type to use
    • Allows power user to resolve performance tradeoffs
  • Iteration through TreeMap guaranteed to be in order, HashMap faster but guarantees no particular order
  • Basic data structures in Java more closely resemble underlying hardware

Abstract Classes

  • Interface inheritance: What (class can do)
  • Implementation inheritance: How (class does it)

Closer Look at Interfaces

  • Interfaces may combine mix of abstract & default methods
    • Abstract methods = what, must be overridden by subclass
    • Default methods = how
  • Every method in interface must be public → public modifier redundant
  • Unless using keyword default, method will be abstract
  • Can provide variables in interfaces → will be public static final
    • final = value can never change → use for constants

Interface Summary

  • Cannot be instantiated
  • Can provide either abstract or concrete/default methods
    • Use no keyword for abstract methods
    • Use default keyword for concrete methods
  • Can provide only public static final variables (no instance variables)
  • Can provide only public methods (no non-public helper methods)

Abstract Classes

  • Intermediate level between interfaces & classes
  • Cannot be instantiated
  • Can provide either abstract or concrete methods
    • Use abstract keyword for abstract methods
    • Use no keyword for concrete methods
  • Can provide variables (any kind)
  • Can provide methods (any kind)
  • abstract methods must be implemented in subclasses
    • Implementations must override ALL abstract methods
  • abstract class does not have to implement/list implemented interface's abstract methods

Summary: Abstract Classes vs Interfaces

  • Interfaces:
    • Primarily for interface inheritance, limited implementation inheritance
    • Classes can implement multiple interfaces
  • Abstract classes:
    • Can do anything interface can do, & more
    • Subclasses only extend one abstract class

Packages

Canonicalization

  • Want ability to provide canonical name for everything
    • Canonical representation: Unique representation for thing
    • Non-canonical: license plate # (can be reused, can change)
    • Canonical: VIN number refers to specific car/motorcycle
  • In Java, attempt to provide canonicity by giving every class a "package name"
    • Package = namespace that organizes classes & interfaces

Packages

  • Address fact that classes might share names:
    • Naming convention: Package name starts w/ website address (backwards)
  • If used from outside, use entire canonical name
  • If used from class in same package → can just use simple name
  • Can use import statement to provide shorthand notation for usage of single class in package
  • Wildcard import: also possible to import multiple classes, but often bad idea
    • Use sparingly b/c contaminates namespace, implicit rather than explicit
    • Wildcard importing from multiple packages → may result in conflicts

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